1. Field of the Invention
This invention relates to extensible booms for cranes and the like. More specifically, the invention concerns a suspended boom with movable sections that can be readily extended and these sections can be pinned together in desired positions. The invention also pertains to the method of changing the length of the extensible boom.
2. Description of the Prior Art
At one time, crane booms were transported to a job site in sections and it was necessary to put the sections together to form a boom of a desired length. If a longer length was desired, it was necessary to put additional sections in the boom and the process of lengthening the boom involved a considerable amount of time and effort by a crew of workmen. If, after assembling the boom to the desired length, the boom was found to be too long to raise in the place where it was needed, more time was lost while the boom sections were changed, as required, to shorten the length of the boom.
Extensible booms were developed to facilitate the rapid extension or retraction of a boom at a work site. One common type of extensible boom utilizes hydraulic cylinders for extending the movable boom sections. These booms are heavy because the movable sections are supported as cantilevers and thus, are subjected to large stresses. Heavy boom sections limit the reach, the height, and the lifting capacity of booms. An example of such a cantilever boom is shown in U.S. Pat. No. 2,684,159 of Oldenkamp.
Cable support systems have been developed for extending, retracting, and suspending extensible booms and these systems overcome some of the disadvantages associated with cantilever booms. Since the boom tip is supported by a suspension cable, a truss is formed by the cable, a cable support and the boom. This tends to eliminate internal bending stresses from the boom because the boom is a compression member within the truss. U.S. Pat. No. 2,819,803 of Obenchain shows an extensible boom that can be maintained at a substantially constant boom angle while the boom is extended or retracted. The boom tip is supported by pennant lines that are extended as the length of the boom increases and these lines are shortened as the boom is retracted. The pennant lines and the lines for extending and retracting the boom are compensating. A lock mechanism that includes a latch plate for engaging selected teeth in longitudinal rows is provided to retain the boom in any selected position of its extensible length for performing work. The lock mechanism is automatically released in response to control by a crane operator when it is desired to change the length of the boom.
Extensible booms, being supported at the boom tip by a suspension cable that is simultaneously compensating with an extension-retraction mechanism, are also shown in U.S. Pat. Nos. 3,029,954; 3,194,413; 3,308,967; 3,341,629; 3,426,917; 3,465,899; and 3,622,013. This type of boom suspension makes it difficult to extend the boom, particularly at a low boom angle near a horizontal position, because the load applied by the suspension cable to the tip of the boom places a force on the tip boom section that reacts axially of the boom against the boom extension mechanism.
U.S. Pat. No. 2,999,600 of Gates shows an extensible boom with a compensating cable suspension system supporting the boom at the boom tip and at the outer end of the lowermost boom section. A compensating cable suspension system supporting an extensible boom at the outer ends of each of four telescoping sections is shown in U.S. Pat. No. 3,534,867 of Johnston et al. Both of these systems apply loads to the outer ends of the movable sections and these loads tend to resist extension of the boom. Furthermore, since these booms are supported at their tips and at points intermediately of their length, undesirable bending stresses can develop within the boom sections near the points of intermediate support.
U.S. Pat. No. 3,845,866 of Eucken shows a telescopic crane jib with sections being fastened together by a locking bolt that is controlled by a fluid operated power cylinder. The jib sections are, extended and retracted in a vertical position by a fluid pressure operated cylinder. A catch mechanism holds an upper section to a lower section, while the fluid pressure operated cylinder is lowered, taking with it the other telescopic parts. In the lower positions, the lower end of the upper telescopic part can be locked to the upper end of the next telescopic part by a locking bolt that is controlled by the fluid operated power cylinder. The catch mechanism is released and the cylinder is raised, pushing the extended upper part of the housing until the upper end of the next part engages the catch mechanism.
Problems are encountered when pinning the sections of a crane boom together in positions other than vertical. In such positions, vertical bending moment is applied to the sections, causing some misalignment between the pins and the pin receiving holes. Due to such misalignment, it is possible that the pins will not be seated in the receiving holes. With this possibility, some crane operators want to set the pins while the boom is in a substantially horizontal position and then walk along each side of the boom to make certain that all pins are set before elevating the boom to a substantial height. Other crane operators, when using a boom with sections that have been pinned together in an upwardly inclined position, are satisfied if they can see from the ground that the pins are set.